High-intensity arc lamp

ABSTRACT

A short arc gas discharge lamp includes a solid ceramic body provided with a reflector-shaped surface extending from one end face into the body. A thermally conductive tube inserted in an aperture extending through the other end face of the ceramic body supports an anode opposite to a cathode. The cathode is mounted adjacent to a window of the lamp and in predetermined relationship to the one end face of the body so that the point of highest intensity of a gas discharge within the lamp will be located at the focal point of the reflector surface. In one embodiment of the lamp, the reflector-shaped surface of the ceramic body is metallized to provide a reflective surface, whereas in a second embodiment the reflector-shaped surface is provided with a smooth glazed coating that is suitable for direct reception of a metallic reflective coating without the use of intermediate metallizing steps such as plating and polishing.

llited States Patet 191 McRae et al.

1 1 May 1, 1973 [54] HIGH-INTENSITY ARC LAMP [75] Inventors: Russell C. McRae, Cupertino; William R. Stuart, San Carlos, both of Calif.

[63] Continuation-impart of Ser. No. 109,527, Jan. 25,

1971 abandoned.

[52] Cl 313/113 240/103 313/184 discharge within the lamp will be located at the focal 51] Int. Cl ..1'm1,-/ i, i1 d1 k i of the reflect surface one embodiment 9 the lamp, the reflector-shaped surface of the ceramic [58] Field of Search ..313/1l3, 220, 214,

body is metallized to provide a reflective surface, 313/205,184, 240/103 R whereas in a second embodiment the. reflector-shaped [56] References Cited surface is provided with asmooth glazed coating that is suitable for direct reception of a metallic reflective UNITED STATES PATENTS coating without the use of intermediate metallizing 3,418,507 12/1968 Young ..313 113 steps Such as Plating and Pdishing' 3,450,924 6/1969 Knochel et al. .....3l3/l13 3,495,118 2/1970 Richter ..313 113 13 Clams 8 Drawmg FOREIGN PATENTS OR APPLICATIONS 767,519 2/1957 Great Britain ..313/1l3 Q 5 ,y I 1 27 V\\\ 5 2 l 0 b 2 4 Primary Examiner-John Kominski Att0rneyChester E. Martine, Jr.

[57] ABSTRACT A short arc gas discharge lamp includes a solid ceramic body provided with a reflector-shaped surface extending from one end face into the body. A thermally conductive tube inserted in an aperture extending through the other end face of the ceramic body supports an anode opposite to a cathode. The cathode is mounted adjacent to a window of the lamp and in predetermined relationship to the one end face of the body so that the point of highest intensity of a gas PATENTEB 11975 I 3.781.133

sum 1 [1F 2 2B ig/3 A INVENTORS RUSSELL (INC RAE 82 WILLIAM R.STUART HIIHHHHHIHllHHllll lll so k? HIGH-INTENSITY ARC LAMP RELATED APPLICATION This Application is a continuation-in-part of patent application Ser. No. 109,527 filed on Jan. 25, 1971 and now abandoned, in the name of the present inventors for An Improvement to a High Intensity Arc Lamp.

BACKGROUND OF THE INVENTION This invention relates to a new and improved gaseous discharge device and more particularly to an improved high-intensity short are lamp structure.

In optical projection systems involving the generation and precisely controlled radiation of long pulses of non-coherent light, such as those used in spectroscopy, microscopy, and solar simulation, in addition to the more conventional projection systems, there is a need for a light source capable of producing the highest possible light flux density, that is, the greatest total amount of light from the least possible volume. The ideal would be a point source of light with unlimited light output.

Of the electrical devices for generating non-coherent light in pulses of substantial duration, gas discharge devices offer the possibility of generating the greatest light flux density. In particular, the light flux density which can be produced by incandescent or luminescent devices is limited by the amount of power that can be concentrated in the solid materials which serve as the light emitters before a change of state from solid to gas occurs in such material, whereas in a gas discharge device no such change of state can occur in the light emitting medium regardless of the concentration of power.

The amount of power which can be concentrated in a gas discharge may be maximized by decreasing the spacing between the electrodes of the device and increasing the pressure of the gaseous medium, the voltage at which the discharge operates, and the current 7 carried by the are that extends between the .electrodes.

It has been found that for any given voltage and current, the greatest light flux density will be obtained when the electrode spacing and gas pressure are adjusted to produce an arc discharge which is roughly spherical (that is, the length of the arc is approximately equal to its transverse dimensions). In this mode of operation, the electrode spacing is less than two centimeters and usually less than one centimeter. Arc discharge devices designed to operate in this mode are called short arc" devices to distinguish them from other forms of arc discharge such as medium are" and long arc" devices which may produce larger total quantities of light but at a much lower light flux density.

This invention is an improvement in the short are lamp disclosed in U.S. Pat. No. 3,502,929 which issued on Mar. 24, 1970 to John F. Richter. This prior art lamp comprises a sealed envelope, a short, annular portion of which is ceramic to provide electrical insulation between a cathode and an anode which are spaced apart a distance less than 2 centimeters to define a short are gap therebetween. The envelope also houses an ionizable gas typically under approximately standard atmospheres of pressure. A sapphire window forms one end of the envelope and a reflector is attached to the other end of the envelope, either as part of the envelope or separate therefrom.

In the usual embodiment of this prior art lamp, the anode is suspended on the axis of the lamp adjacent to the window. As much as percent of the energy of the discharge is converted into heat at the anode. This heat must be dissipated through the structure supporting the anode and then through flanges thermally connected to the support structure. The support structure is thin because light must pass by it. The flanges are thin to allow rapid dissipation of the heat in the critical area of the seal at the window. While highly effective at power consumptions up to approximately watts, at powers above this level heat cannot be properly dissipated and the seals break down. These seals themselves must be thin to allow for proper expansion as they absorb heat.

Reversing the positions of the cathode and anode so that the cathode is adjacent to the window places the anode in the base of the lamp wheremore massive structures can be used to provide better heat dissipation. In the past, however, attempts to reverse the positions of the cathode and anode were limited by the requirement that the prior reflectors be thermally insulated from the anode. Moreover, prior attempts to place the cathode adjacent to the window with the anode adjacent to the base gave rise to a new problem which offset the advantages of higher heat dissipation. In particular, the point of highest intensity in the arc discharge depends on the position of the cathode (not the anode) and must be at the focal point of the reflector to give the greatest light flux density. Small errors in such position greatly reduce the light flux density. When the cathode is in the base, the positioning of the cathode and therefore the positioning of the point of highest intensity relative to the focal point of the reflector is fairly easy because the reflector and the cathode are in the same assembly. However, prior attempts to locate the cathode adjacent to the window were dependent upon maintaining close tolerances in a number of parts, including the anode assembly, the reflector assembly and in the brazed joints between the ceramic annular portion and such assemblies. Because of problems encountered in maintaining such tolerances, the usual prior art lamp has the anode adjacent to the window to obtain the most light flux density even though the maximum power-handling capacity of the lamp is thereby limited.

SUMMARY OF THE INVENTION Research conducted in an endeavor to provide a lamp capable of producing increased light flux density indicates that the provision of an anode in the base of a solid ceramic envelope having an integral reflector surface formed into one end thereof and a cathode mounted in predetermined relation with respect to such end results in a lamp that can be more accurately assembled and one having improved heat dissipation characteristics. In particular, the improved lamp includes a cathode support structure mounted against the one end of the envelope for mounting the cathode on the axis of the envelope in spaced relation to the anode so that the point of highest intensity in the are discharge is at the focal point of the reflector. The one end of the lamp is sealed by a window mounted against the cathode support structure, whereas a thermally conductive anode support tube extending through the other end of the envelope is pinched off to complete the sealing of the lamp. The tube conducts heat from the anode to a heat radiator provided on the base of the envelope for effectively dissipating heat from the lamp.

In one embodiment, the integral surface is metallized to provide a surface suitable for reception of a metallic reflective coating, whereas in another embodiment, the integral surface is provided with a smooth glazed coating that is covered with a metallic reflective coating.

An object of the present invention is to provide a new and improved high-intensity arc lamp in which a cathode mounted adjacent to a window is precisely positioned relative to a reflector surface.

Another object of the present invention is to provide a lamp with higher acoustic resonance frequency so that the lamp can be modulated at higher frequencies.

An additional object of the present invention is to provide a lamp in which the scintillation of the light beam found in prior art lamps is reduced.

A further object of the invention resides in the provision of improved facilities for dissipating heat from the base of a lamp.

A still further object of the present invention is to provide a light reflector having improved specular reflectance properties while simplifying the manufacture of the reflector.

BRIEF DESCRIPTION OF THE DRAWINGS With these and other objects in mind, the principles of the present invention may be understood by referring to the following description of the preferred embodiment in conjunction with the appended drawings in which:

FIG. 1 is a cross-sectional view of one embodiment of the prior art lamp disclosed in U.S. Pat. No. 3,502,929;

FIG. 1A is a frontal view of the lamp shown in FIG. 1 taken along the line 1A-1A in the direction of the arrows;

FIG. 2 is a cross-sectional view of one embodiment of a high-intensity short are lamp constructed according to the principles of the present invention;

FIG. 2A is a front view of the lamp shown in FIG. 2 taken along the line 2A2A in the directionof the arrows;

FIG. 2B is a rear view of the lamp shown in FIG. 2 taken along the line 28-28 in the direction of the arrows;

FIG. 3 is a rear view of one type of radiator which may be attached to the base of the lamp of this invention;

FIG. 3A is a side view of the radiator shown in FIG. 3 taken along the line 3A-3A in the direction of the arrows; and

FIG. 4 is a cross-sectional view of another embodiment ofa lamp constructed according to the principles of the present invention, illustrating an improved reflector having a glazed and coated metallic surface.

DESCRIPTION OF THE PRIOR ART FIGS. 1 and 1A illustrate an embodiment of the prior art lamp. One end of an annular ceramic section 40, which is made of polycrystalline alumina, is brazed in a ductile metallic (copper, for example) ring 42 which in turn is brazed to a metallic (Kovar or stainless steel, for

example) member 44 of the lamp envelope. The metallic member 44 may be spherical, ellipsoidal or parabolic. The ductile metallic ring 42 serves as a stress relieving portion of the envelope. The inner surface of the member 44 serves as an integral reflector 46. The other end of the annular ceramic section 40 is brazed to a ductile metallic ring 48, which in turn is brazed to one side of a rigid metallic terminal ring 50. The terminal ring 50 is brazed to another ductile metallic ring 52, which in turn is brazed to the flange of a tubular rigid metallic window support 54. As in the case of the ring 42, the ductile metallic rings 48 and 52 serve to relieve stresses. The periphery of a disc-shaped window 56, which may be sapphire, for example, is slightly recessed within and brazed to the window support 54.

A rod-shaped metallic anode 58 (tungsten, for example) is supported along the axis of the annular ceramic section 40 by three triangular, metallic supports 60 which may be of molybdenum, for example. Each support 60 has a notch into which the terminal ring 50 is brazed. The supports 60 provide electrically conductive paths between an anode 58 and the terminal ring 50. Each of the metallic supports 60 is bent into the shape of a spiral for stress relief during high temperature states.

A rod-shaped cathode 62 (e.g., thoriated tungsten) is supported adjacent to the anode 58 on the axis thereof by a metallic cup 64. This cup, which may be Kovar, for example, and which forms a portion of the sealed envelope, is brazed about the periphery of an aperture in the member 44. A copper exhaust tubulation 66 communicates through the cup into the interior region of the envelope. Once the envelope has been filled with xenon, for example, and pressurized, the copper tubulation 66 is pinched off, thereby confining the pressurized gas within the sealed envelope.

DESCRIPTION OF PREFERRED EMBODIMENTS There is illustrated in FIGS. 2, 2A and 23 a high-intensity short arc lamp which includes a ceramic cylinder 10. The cylinder 10 may be of a ceramic such as alumina AD995A manufactured by Coors Porcelain Corporation of Golden, Col., for example. This type of alumina has a near optical surface thus making any necessary final polishing step easier. A reflector surface 1 1 is formed by cutting into one face 19 of the cylinder 10 along a longitudinal axis 10a of the cylinder. The surface 11 is shaped to give the desired beam. The surface 11 is shown having a paraboloidal shape but the reflector may be, for example, spherical or ellipsoidal. The reflector is shown with a longitudinal portion 12 whose significance will be discussed below.

A hole 13 is cut from the bottom (or left-hand side as viewed in FIG. 2) of the reflective surface along the axis 10a of the cylinder 10 through the remainder of the cylinder 10. The hole 13 is comprised of two approximately equal cylindrical portions 14 and 15. The portion 15 is of slightly less diameter than the portion 14. A tube 16 of high thermal conductivity, such as of copper, having an outer diameter equal to the diameter of the hole portion 15 is inserted from a base 10b of the cylinder 10 through the hole portion 15 into the hole portion 14. The tube 16 has a hole 18 cut diametrically through it. As will be explained below, the hole 18 is used in evacuating and filling the lamp. It is necessary that the'hole 18 be within the hole portion 14 to allow for gas evacuation and filling but the precise lineup shown in FIG. 2. is not necessary. In FIG. 2, the hole 18 is shown as being exactly at the left end of the hole portion 14 but the lamp will function equally as well with the hole 18 to the right of the intersection of the hole portion 14 and the hole portion 15 or with the hole 18 extending into the hole portion 15.,An anode 17 of, for example, tungsten is inserted into the end of the tube 16 and sealed by brazing. The base of the anode 17 need not line up exactly with the hole 18 as shown in FIG. 2. The lamp will function properly if the base or left end of the anode 17 is located to the left or the right of the portion shown in FIG. 2 so long as the hole 18 opens into the hole portion 14 and into the interior of the tube 16.

A ring having an outer diameter equal to the outer diameter of the cylinder 10 and an inner diameter slightly less than that of the remaining face 19 of the cylinder 10 is brazed to the face 19. The ring 20 may be of an alloy of iron, nickel and cobalt having a coeffi' cient of thermal expansion approximating that of the ceramic. Such an alloy is sold under the trademark Kovar. Another generally L-shaped ring 21 is shown brazed to the surface of ring 20. The ring 21 has a portion extending from the inside edge of the face 19 outwardly towards the outside edge of the ring 20 where the ring 21 is bent away from the ring 20 to form a horizontal standing edge portion 24. The height of the standing edge portion-24 is a matter of convenience as discussed below. A ring 22 is brazed to the outside of the cylinder 10 and extends to meet the portion 24 of the ring 21. A tungsten inert gas are weld is formed where the standing portion 24 meets the ring 22. The ring 22 serves to protect the front end of the lamp during operation.

Three thin supports 27 are of generally rectangular shape with one corner 28 truncated and may be made of, for example, molybdenum. The truncation is such that an edge 29 is of length approximately equal to the thicknesses of the rings 20 and 21. Each support 27 is brazed to a notch in the ring 20 at the edge 29. The three supports 27 support a rod-shaped metallic cathode 30 of, for example, thoriated tungsten. The anode 17 and the cathode 30 are positioned on the axis 10a of the lamp. The anode l7 and the cathode 30 are spaced apart by less than 2 centimeters, preferably less than one centimeter. The cathode 30 is positioned so that the point of greatest light intensity in the arc discharge is at the focal point of the reflector surface 11.

The lamp envelope includes a disc-shaped window 31 of, for example, sapphire. The window 31 is brazed to a generally L-shaped sealing ring 23 having a portion 32 brazed to the portion 25 of the ring 21. The point at which the arc weld is made between the rings 21 and 22 must be thermally isolated from the window. For this reason, it is convenient to have the ring 22 and the portion 24 of the ring 21 extend beyond the outer surface of the window 31. In addition to aiding the welding process, this allows the ring 22 to provide protection to the window 31.

In preparing the surface 11 of the reflector prior to any assembly, the surface may be metallized in any well-known manner and then polished to be a reflector.

By way of example, the metallized surface 11 may be provided by coating the shaped ceramic surface by the molymanganese process to provide a layer to which a layer of plated copper will adhere. The copper layer is polished to provide a smooth surface for reception of an evaporated coating of a reflector material, such as 1 silver or rhodium, for example.

The longitudinal surface 12 is normally not metallized. In particular, it may be understood that while a lamp of this type will operate at very low voltage, such as 20 volts, once in operation, high voltages, such as 20,000 volts, must be applied across the gap to start the lamp. These voltages are typically supplied by a high frequency RF. source (not shown). To help prevent current from passing along the reflector surface 11 and across the gap formed by the hole 13 to the anode 17, the longitudinal surface 12 is normally left unmetallized. This means that the reflector surface 11 is not in electrical contact with either the cathode 30 or the anode 17.

After assembly, the lamp is evacuated through the tube 16 and then gas-filled with, for example, xenon to a pressure of approximately 25 atmospheres. The tube 16 is then sealed at a pinch off 35.

A plate 36 of, for example, copper is brazed to the base 10b of the ceramic cylinder 10. A hole 37 in the center of the plate 36 accommodates the tube 16. The plate 36 is in thermal contact with the tube 16. As mentioned, as much as percent of the energy of the discharge is converted to heat at the anode of a short are lamp. This heat travels readily from the anode 17 through the tube 16 into the plate 36. The simple construction of the base 10b of this lamp makes it possible to attach radiators of relatively simple construction. Such a radiator is shown in FIGS. 3 and 3A. The radiator consists of a cylinder 81, a ring 82 spaced radially from the cylinder 81, and a serpentine fin 80 placed between and attached to the cylinder 81 and the. ring 82. The tops of both the fin 80 and the ring 82 are flush with the top of the cylinder 81. However, as shown in FIGS. 3A, the cylinder 81 extends longitudinally below the fin 80 while the tin 80 extends longitudinally below the ring 82. Three holes are threadedly bored into the base 10b of the lamp. Bolts (not shown) are then inserted through holes 76 in the cylinder 81 and into the holes 75 in the base 10b. The simple construction of the base 10b allows the outer surface of the plate 36 to be in thermal contact with the entire upper surface of the radiator. A hole 77 in the center of the cylinder 81 ac commodates the tube 16 with its pinch seal 35.

In some applications using a short are lamp, it is desirable to modulate the current across the arc gap, thus modulating the light produced by the lamp. If the modulation frequency is at or near an acoustic resonance frequency of the lamp, the gas molecules in the lamp will oscillate. This causes the pressure at the arc gap to vary from near zero to maximums far greater than the normal operating pressure of the lamp. At these maximums the current in the arc gap can no longer be maintained and the lamp will be extinguished. It is desirable to have the lowest of the lamps acoustic resonance frequencies be as high as possible so that it will exceed any modulating frequency which might be used. This lowest acoustic resonance frequency increases as the volume of gas decreases.

Both the present invention and the prior arc lamp consist of a pair of coupled cavities. The lowest acoustic resonance frequency is a complex function of the resonance frequencies of each of these coupled cavities. However, the major contribution comes from the cavity in which the arc gap is located. The simple structure of the present invention allows the distance from the bottom (or inside surface) of the window 31 to the left side of the reflector surface 11 to be much less than in the prior art lamp. Thus, for the same aperture size and shape of reflector, the volume of gas in the chamber which includes the arc is smaller in the present invention than in the prior art.

The larger volume of the prior art lamp allowed the gas within the lamp to circulate freely. The smaller volume of the lamp in the present invention yields greater gas stability. As a result, the temperature at any one point in the lamp remains relatively constant during operation. However, in the prior art lamp the temperature at any given point changed as the gas circulated. This resulted in an undesirable scintillation effect in the lamps beam. The greater stability of the gas in the present invention reduces this scintillation effect.

While it was feasible to place the anode in the base of the prior art lamp, this resulted in more likely error in the positioning of the point of highest intensity in the arc gap relative to the focal point of the reflector. If this was to be done with the desired accuracy, all parts had to be machined with sufficient accuracy to insure that the focal point of the reflector was at a precise distance from the point of support of the cathode. In the present invention, the ceramic cylinder can be cut to the desired shape with extremely accurate tolerances at relatively low cost. The supports for the cathode are then indexed directly to the surface of the cylinder at the face 19. This allows for extremely accurate placement of the point of highest intensity relative to the focal point of the reflector.

Referring to FIG. 4, a second embodiment of the lamp 91 of the present invention is shown, including the ceramic cylinder along with a similar anode l7, cathode 30, cathode supports 27 and structure for supporting the window 31. The ceramic cylinder may, for example, be formed from alumina AD94A manufactured by the Coors Porcelain Corporation. In the lamp 91 of the second embodiment, the ceramic cylinder 10 is provided with a surface 92 that is shaped so that upon reception of a coating 93 and a reflective layer 94, a reflector 95 having a desired shape, such as ellipsoidal, for example, is formed. For example, the surface 92 may be formed by pressing, such as isostatically, unfired ceramic powders against a suitably shaped mandrel (not shown). The coating 93 is capable of withstanding the relatively high temperatures of the gas discharge and is preferably a high temperature, smooth glazed coating having relatively uniform thickness. In particular, the coating 93 may be formed by applying to the surface 92 frit comprising a vehicle and a glazing compound. The glazing compound may be potassium aluminum silicate, such as Kingsman feldspar, for example, or a standard blend of Custer feldspar, Kaolin and alumino silicate sold as glaze [6-7 by Coors Porcelain Corporation of Golden, Col. Other alumino silicate or borosilicate glazes may be used so long as the glazed coating 93 has a coefficient of thermal expansion comparable to that of the surface 92, has a glossy surface and does not soak into the surface 92 during firing.

The frit may be applied, such as by spraying, to the entire surface 92 in sufficient quantity and uniformity of thickness to provide the completed coating 93 with a uniform and sufficient thickness so as to avoid production of a matt finish while avoiding rippling resulting from excessive thickness. The thickness of the completed coating 93 is preferably from 2 to 6 mils. The cylinder 10 is then placed with the face 19 on a horizontal surface and the glazing compound is fired at a temperature in excess of the melting point of the glazing compound (such as at 1,500 i 50C), to fuse the glazing compound and provide the glazed coating 93 with the relatively uniform thickness devoid of ripples, crazing and pinholes.

The reflective layer 94 is then applied to the glazed coating 93, such as by evaporating silver onto the coating 93 to provide the reflector 95 with improved specular reflectance characteristics.

It may be understood that the use of the glazed coating 93 renders unnecessary the steps of the metallizing process. Thus, such steps as plating and polishing are not necessary to produce the lamp 91, and as a result, problems relating to eccentricity of the reflector 95 relative to the axis 10a as well as scratching, etc., are eliminated.

It is to be understood that only preferred embodiments of the present invention have been specifically illustrated and described, and variations may be made thereto without departing from the invention, as defined in the appended claims.

What is claimed is:

1. An arc lamp comprising an opaque dielectric member forming the sidewall and base end portion of said lamp, a transparent wall portion forming an optical window adjacent the end of said lamp opposite said base end, the window end of said opaque dielectric member having a concave surface on which a reflector is formed, an anode and a cathode electrode each having one end positioned within the space formed by said concave surface, said one end of said electrodes being spaced apart to form an arc gap, first metallic means sealing one of said electrodes and said window to said window end of said opaque dielectric member, said first metallic means forming an electrical lead for said one electrode, the base end of said opaque dielectric member having an aperture, and second metallic means sealing the other of said electrodes in said aperture and forming an electrical lead for said other electrode.

2. The arc lamp of claim 1 wherein said reflector is configured as a surface of revolution about an axis.

3. The arc lamp of claim 2 wherein said electrodes are elongate with the long axes thereof coinciding with the axis of the reflector.

4. The arc lamp of claim 1 wherein said opaque dielectric member is ceramic.

5. The arc lamp of claim 4 wherein said ceramic member is made of alumina.

6. The are lamp of claim 1 wherein said reflector comprises a polished portion of said concave surface.

7. The are lamp of claim 1 wherein said reflector comprises a metallized portion of said concave surface.

8. The arc lamp of claim 1 wherein said reflector comprises a glazed coating on said concave surface.

9. The are lamp of claim 8 wherein said glazed coating has a thickness of from two to six mils.

10. The are lamp of claim 1 wherein said opaque dielectric member is cylindrical, said first metallic means comprises an annular structure disposed coaxially with and exteriorly of said opaque dielectric member, and said one electrode is mounted coaxially with said annular structure.

11. The are lamp of claim 1 wherein said second metallic means comprises tubulation, a portion of said other electrode is disposed within the bore of said tubulation, and said tubulation is apertured to provide a path for evacuation of air therethrough from said envelope.

12. The are lamp of claim 1 wherein said opaque dielectric member is cylindrical and said first metallic means comprises a first annular ring which abuts the window end of said opaque dielectric member and is coaxial therewith, said first annular ring having an outer diameter equal to the outer diameter of said opaque dielectric member, a second annular ring which has a generally L-shaped rim cross-section, one arm of said rim abutting said first annular ring, said first and second annular rings being sealingly joined together at said abutment, a third annular ring which abuts said second annular ring and is sealingly joined thereto, said third annular ring supporting and sealingly joined to said window, and a fourth annular ring which surrounds a portion of said opaque dielectric member adjacent the abutment of said opaque dielectric member with said first annular ring, said fourth annular ring substantially surrounding said first, second and third annular rings and said window.

13. The are lamp of claim 1 wherein a metallic member is affixed to the exterior surface of the base end of said opaque dielectric member, said metallic member being in contact with said second metallic means to provide a thermally conductive path for the removal of heat from said other electrode. 

1. An arc lamp comprising an opaque dielectric member forming the sidewall and base end portion of said lamp, a transparent wall portion forming an optical window adjacent the end of said lamp opposite said base end, the window end of said opaque dielectric member having a concave surface on which a reflector is formed, an anode and a cathode electrode each having one end positioned within the space formed by said concave surface, said one end of said electrodes being spaced apart to form an arc gap, first metallic means sealing one of said electrodes and said window to said window end of said opaque dielectric member, said first metallic means forming an electrical lead for said one electrode, the base end of said opaque dielectric member having an aperture, and second metallic means sealing the other of said electrodes in said aperture and forming an electrical lead for said other electrode.
 2. The arc lamp of claim 1 wherein said reflector is configured as a surface of revolution about an axis.
 3. The arc lamp of claim 2 wherein said electrodes are elongate with the long axes thereof coinciding with the axis of the reflector.
 4. The arc lamp of claim 1 wherein said opaque dielectric member is ceramiC.
 5. The arc lamp of claim 4 wherein said ceramic member is made of alumina.
 6. The arc lamp of claim 1 wherein said reflector comprises a polished portion of said concave surface.
 7. The arc lamp of claim 1 wherein said reflector comprises a metallized portion of said concave surface.
 8. The arc lamp of claim 1 wherein said reflector comprises a glazed coating on said concave surface.
 9. The arc lamp of claim 8 wherein said glazed coating has a thickness of from two to six mils.
 10. The arc lamp of claim 1 wherein said opaque dielectric member is cylindrical, said first metallic means comprises an annular structure disposed coaxially with and exteriorly of said opaque dielectric member, and said one electrode is mounted coaxially with said annular structure.
 11. The arc lamp of claim 1 wherein said second metallic means comprises tubulation, a portion of said other electrode is disposed within the bore of said tubulation, and said tubulation is apertured to provide a path for evacuation of air therethrough from said envelope.
 12. The arc lamp of claim 1 wherein said opaque dielectric member is cylindrical and said first metallic means comprises a first annular ring which abuts the window end of said opaque dielectric member and is coaxial therewith, said first annular ring having an outer diameter equal to the outer diameter of said opaque dielectric member, a second annular ring which has a generally L-shaped rim cross-section, one arm of said rim abutting said first annular ring, said first and second annular rings being sealingly joined together at said abutment, a third annular ring which abuts said second annular ring and is sealingly joined thereto, said third annular ring supporting and sealingly joined to said window, and a fourth annular ring which surrounds a portion of said opaque dielectric member adjacent the abutment of said opaque dielectric member with said first annular ring, said fourth annular ring substantially surrounding said first, second and third annular rings and said window.
 13. The arc lamp of claim 1 wherein a metallic member is affixed to the exterior surface of the base end of said opaque dielectric member, said metallic member being in contact with said second metallic means to provide a thermally conductive path for the removal of heat from said other electrode. 